A display device used for an electronic device (e.g., mobile phone, smartphone, car navigation system, personal computer, ticket vending machine, and ATM) has a structure in which a touch panel and a transparent protective sheet are disposed at a given gap (formed by an air layer) from a display panel (e.g., liquid crystal display panel or organic EL display panel). However, the structure in which such a gap is provided (e.g., a structure in which a gap is formed between the display panel and the transparent protective sheet, or between the display panel and the touch panel, or between the touch panel and the transparent protective sheet) has a problem in that loss of light by reflection may occur to a large extent due to the difference in refractive index between the touch panel or the transparent protective sheet and the air layer, and good visibility may not be obtained. Therefore, a method that fills the gap with a transparent photocurable resin or thermosetting resin and cures the transparent photocurable resin or thermosetting resin, or a method that fills the gap with a transparent pressure-sensitive adhesive sheet or the like has been used in order to improve visibility while improving the strength of the protective panel to prevent a situation in which breakage and scattering occur due to an impact (see Patent Literature 1 to 8, for example).
Such a transparent filling material is required to exhibit reworkability that ensures that, when the touch panel is bonded to the screen of the display device through the transparent filling material, and bonding failure or the like has been found, the transparent filling material can be removed from the touch panel and/or the display device, and the touch panel and/or the display device can be recycled.
However, the method that fills the gap with a transparent photocurable resin or thermosetting resin and cures the transparent photocurable resin or thermosetting resin has a problem in that adhesion to the adherend increases, and reworkability cannot be obtained. Moreover, the display panel may be deformed due to the cure shrinkage of the resin, or air bubbles may be easily mixed when filling the gap with the curable resin.
The method that fills the gap with the transparent pressure-sensitive adhesive sheet can prevent the deformation of the display panel, and ensure reworkability.
However, the known method that fills the gap with the transparent pressure-sensitive adhesive sheet cannot achieve both easy reworkability and high tackiness in a satisfactory manner.